Alumina is widely used as a material for substrates of cutting tools due to its excellent mechanical properties and thermal resistance. A TiN coating layer is formed on the surface of an alumina cutting tool to improve surface roughness, reduce cutting resistance and prevent adhesion of workpiece on the cutting tool. However, the TiN coating layer has a low hardness and thus may be easily worn out due to the friction occurring during a cutting operation. When cutting hard materials such as a cast iron, the TiN coating layer tends to wear out more easily. Further, the TiN coating layer is easily oxidized at above 1000° C. under an atmospheric exposure. Thus, when the cutting tool performs a high-speed cutting, the TiN coating layer, which is subjected to a high temperature, tends to be easily oxidized and peeled off from the substrate.
The removal of the TiN coating layer causes the cutting edges to wear out rapidly. Further, it causes an increase in cutting resistance, thereby accelerating the abrasion of the cutting tool. This shortens the service life of the cutting tool.